This invention generally concerns optical amplifiers, and is particularly concerned with a method of operating a Raman-type amplifier to reduce noise generated by four-wave mixing.
Optical amplifiers for amplifying photonic signals transmitted through optical fiber networks are well known in the art. Such amplifiers are used to extend transmission distances and to compensate for losses from various network elements. Presently, there are two known types of optical amplifiers, including erbium-doped fiber amplifiers (EDFAs), and Raman amplifiers.
EDFAs typically comprise a pump laser whose output is optically coupled to the input of two or more serially connected coils of erbium-doped optical fiber. In operation, the output of the pump laser excites the atoms of the erbium-dopant within the serial connected coils. These excited atoms release their excess energy in proportion to the strength of the incoming optical signal, which results in an amplified output. By contrast, Raman amplifiers achieve amplification without the need for specially erbium-doped optical fibers; fibers with conventional dopants may be used.
In such an amplifier, the output of a pair of orthogonally polarized pump-diode lasers provide backward propagating pump power in the gain fiber. Forward-propagating signals achieve gain in the fiber because higher energy (shorter wavelength) pump photons scatter off the vibrational modes of the fiber""s lattice matrix and coherently add to the lower-energy (longer wavelength) signal photons.
Raman amplifiers may be one of two types, depending upon the source of the gain fiber used therein. Distributed Raman amplifiers advantageously use the transmission fiber itself as the gain fiber. By contrast, discrete Raman amplifiers employ their own gain fiber which is added to the transmission fiber of the network. While the dopant used in the gain fiber of a discrete Raman amplifier is typically the same as used in the transmission fiber (i.e., germanium), the Raman gain fiber is usually doped with higher concentrations of germanium than conventional transmission fiber and is designed to operate with a decreased fiber effective area in order to provide a high non-linear coefficient to the resulting gain fiber. The resulting reduction in non-linear properties of the fiber allows the gain fiber to transmit at a higher bandwidth.
Unfortunately, the high concentration of dopant in combination with the decreased fiber effective area does not eliminate all non-linearities, and makes such discrete Raman amplifiers more vulnerable to a type of noise known as four-wave mixing (FWM). FWM can occur when two or more frequencies of light propagate through an optical fiber together because of nonlinear susceptibilities of the optical fiber. As a result of FWM, light is generated at new frequencies using optical power from the original signals. The end result is an interfering tone in the optical signal being transmitted. Generation of such light through FWM has negative implications in communications systems because the generated light is noise with respect to the communications signal and the power of the communications signal is reduced. Accordingly, the signal to noise ratio can be greatly reduced by FWM phenomenon. It is therefore desirable to reduce the effects of FWM in fiber amplifiers.
It is an object of the invention to reduce noise and increase performance of fiber amplifiers. To achieve this and other objects, a first aspect of the invention is a method for amplifying optical signals with a Raman amplifier of the type having an optical fiber coupled to a source of pump light comprising the steps of correlating a signal gain level to a dispersion value of the optical fiber that corresponds to a local minimum power level of a cross-talk signal generated by four wave mixing, and transmitting the optical signals through the optical fiber while admitting sufficient pump light into the fiber from the source to attain the selected Raman gain level.
A second aspect of the invention is a fiber amplifier, comprising a source of pump light at an intensity corresponding to a selected gain level, and a gain fiber coupled to the source of pump light and having an input end and an output end for receiving and transmitting optical signals. A dispersion value of the gain fiber corresponds to a local minimum power level of a cross-talk signal generated by four wave mixing at the selected gain level.